The present invention relates to analog circuits, and more particularly, to a CMOS amplifier with utility for optoelectronic receivers.
With increasing data rates in electronic systems, it is expected that optical interconnects (optical fibers) may in the near future replace wire interconnects at the board-to-board and chip-to-chip level. For example, a computer system such as that illustrated in FIG. 1 may comprise one or more boards 102 and memory hierarchy 104 that exchange data packets over optical interconnects 106. These packets may be routed via switch 108, or perhaps the various integrated circuits may be directly connected to one another. Each board 102 may comprise one or more microprocessors.
In many applications, a photodetector provides an electrical signal indicative of a received optical signal. A typical small-signal model for a photodetector is provided in FIG. 2, comprising small-signal current source 202 and parasitic capacitor 204. The small-signal current provided by current source 202 is representative of the received optical signal. The output signal is provided at output port 206, which for many applications is connected to the input port of a transimpedance amplifier (not shown). Transimpedance amplifiers provide a small-signal output voltage signal in response to a small-signal input current signal.
In telecommunication applications, the received optical signals are typically very small due to attenuation in optical fibers, which may be hundreds of kilometers long. Consequently, a primary goal for transimpedance amplifiers for long haul communications is to provide high transimpedance with low noise amplification, while attaining as large a bandwidth as practical.
However, at the board-to-board and chip-to-chip level, such as the computer system of FIG. 1, attenuation is relatively negligible, and the received signals are typically orders of magnitude larger than for the case of long haul optical communication systems. In such short haul optical applications, it is desirable for economic and high-bandwidth reasons for transimpedance amplifiers to be integrated with other circuits on an integrated circuit die, such as input-output chips on boards 102 or switch 108, and perhaps on a microprocessor itself. Accordingly, for such applications, it is desirable to provide CMOS (Complementary Metal Oxide Semiconductor) transimpedance amplifiers having an acceptable transimpedance over a relatively large bandwidth, such as, for example, 20 Gbps, with relatively low power dissipation, and with good power supply noise rejection.